Exhaust gas from an internal combustion engine, such as automobile engine, contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC), etc, which is generally released into the atmosphere after purification with an exhaust gas purifying catalyst capable of oxidizing CO and HC, and at the same time, reducing NOx. As a representative exhaust gas purifying catalyst, there is known a three-way catalyst where a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) is supported on a porous metal oxide support such as γ-alumina.
The metal oxide support may be formed of various materials, but in order to obtain a large surface area, alumina (Al2O3) has been generally used. However, in recent years, in order to accelerate the purification of exhaust gas by using chemical properties of the support, it has been proposed to use various other materials such as ceria (CeO2), zirconia (ZrO2) and titania (TiO2) which are optionally combined with alumina.
Regarding the chemical properties of the catalyst support, it is known that when rhodium is supported on an alumina particle, which is generally used as the catalyst support, rhodium forms solid-dissolution with alumina during the use thereof, and thereby the catalytic activity is decreased. On the other hand, when zirconia is used as a support for rhodium, such a problem due to forming of solid dissolution can be eliminated, and thereby high catalytic activity inherent in rhodium can be utilized.
With respect to zirconia as a catalyst support, it is known that heat resistance of zirconia can be improved by adding an element selected from the group consisting of an alkali metal, an alkaline earth metal and a rare earth to zirconia (see, for example, Japanese Unexamined Patent Publication No. 2004-275919). Japanese National Patent Publication No. 2002-518171 discloses that good exhaust gas purifying performance is obtained by loading rhodium on a rare earth-doped zirconia catalyst support particle. Furthermore, Japanese Unexamined Patent Publication No. 2002-282692 discloses that a lanthanoid element present in zirconia brings about an anchor effect, i.e., an effect of preventing the movement of rhodium on the zirconia surface, whereby sintering of rhodium is suppressed.
As described above, the lanthanoid element present in a zirconia particle not only improves the heat resistance of the zirconia particle, but also suppresses sintering of rhodium by preventing the surface movement of rhodium supported on the zirconia particle.
The present inventors have found that affinity between a rare earth oxide and rhodium enables the rare earth oxide to prevent the movement of rhodium on a zirconia particle surface, and that the affinity is provided by causing an electron of rhodium to be partially coordinated in the empty electron orbit of the rare earth element. The present inventors have found that the affinity between a rare earth oxide and rhodium, at the same time, allows the rhodium to develop on the surface of the rare earth-doped zirconia particle in a highly dispersed state during production and use of the catalyst.
It is preferable to load a noble metal on a catalyst support particle in a highly dispersed state in order to increase the surface area of the noble metal having catalytic activity. However, as described above, in the case of loading rhodium on a rare earth metal oxide, an electron of rhodium is partially coordinated in the empty electron orbit of a rare earth element. Accordingly, when the size of individual rhodium particles becomes excessively small, shortage of an electron of the rhodium particle in the portion contacting with the rare earth oxide may reach the rhodium particle surface, and thereby bring about a state, where the rhodium particle surface lack electrons, i.e., a state where the rhodium surface is relatively oxidized. Since rhodium exerts its activity in the metal state, it is not preferred, in terms of catalytic activity, that the rhodium surface is in an oxidized state.
Under these circumstances, the present invention provides, by using a rare earth oxide, a catalyst support particle enabling appropriate dispersion of rhodium on a catalyst support surface, while preventing movement and sintering of rhodium on the catalyst support surface. Furthermore, the present invention provides a production process of the catalyst support particle, and an exhaust gas purifying catalyst obtained by loading rhodium on such a catalyst support particle.